Carbon fiber reinforced nickel matrix composite having an intermediate layer of metal carbide

ABSTRACT

COMPOSITE ARTICLES HAVING A NICKEL BASE METAL MATRIX REINFORCED WITH A PLURALITY OF HIGH STRENGTH, HIGH MODULUS CARBON FIBERS HAVING A THIN, INTERMEDIATE LAYER OF A METAL CARBIDE DISPOSED ON THE FIBER SURFACES. SUCH COMPOSITES ARE CHARACTERIZED BY IMPROVED THERMAL CYCLING CHARACTERISTIC AND ARE PRODUCED BY IMMERSING A PLURALITY OF HIGH STRENGTH, HIGH MODULES CARBON FIBERS IN A MOLTEN METAL BATH CONTAINING AN ALLOY CONSISTING ESSENTIALLY OF AT LEAST ONE METAL CAPABLE OF REACTING WITH CARBON TO FORM A METAL CARBIDE AND AT LEAST ONE ACID-SOLUBLE METAL WHICH DOES NOT REACT WITH CARBON TO FORM A METAL CARBIDE AND WHICH ACTS AS A VEHICLE OR CARRIER FOR THE CARBIDE-FORMING METAL; SOAKING THE FIBERS IN SAID MOLTEN METAL BATH FOR A TIME SUFFICIENT TO EFFECT REACTION BETWEEN THE FIBERS AND THE CARBIDE-FORMING METAL TO PRODUCE A METAL CARBIDE COATING ON THE SURFACE OF THE FIBERS; REMOVING UNREACTED METAL FROM THE METAL CARBIDE COATED FIBERS SO-PRODUCED; AND INCORPORATING THE FIBERS INTO A NICKEL BASE METAL MATRIX.

United States Patent 3,796,587 CARBON FIBER REINFORCED NICKEL MATRIX COMPOSITE HAVING AN INTERMEDIATE LAYER OF METAL CARBIDE Raymond Vincent Sara, North Olmsted, Ohio, assignor to Union Carbide Corporation, New York, N.Y. No Drawing. Filed July 10, 1972, Ser. No. 270,209 Int. Cl. C23c 1/04, N US. Cl. 117-71 R 40 Claims ABSTRACT OF THE DISCLOSURE Composite articles having a nickel base metal matrix reinforced with a plurality of high strength, high modulus carbon fibers having a thin, intermediate layer of a metal carbide disposed on the fiber surfaces. Such composites are characterized by improved thermal cycling characteristics and are produced by immersing a plurality of high strength, high modulus carbon fibers in a molten metal bath containing an alloy consisting essentially of at least one metal capable of reacting with carbon to form a metal carbide and at least one acid-soluble metal which does not react with carbon to form a metal carbide and which acts as a vehicle or carrier for the carbide-forming metal; soaking the fibers in said molten metal bath for a time sufficient to effect reaction between the fibers and the carbide-forming metal to produce a metal carbide coating on the surface of the fibers; removing unreacted metal from the metal carbide coated fibers so-produced; and incorporating the fibers into a nickel base metal matrix.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to composite articles having a nickel base metal matrix reinforced with a plurality of high strength, high modulus carbon fibers having a thin, intermediate layer of a metal carbide disposed on the fiber surfaces.

(2) Description of the prior art As a result of the rapidly expanding growth of the aircraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and extraordinary combination of physical properties. Thus, materials characterized by high strength and stiffness, and at the same time of light weight, were required for use in such applications as the fabrication of aircraft structures, re-entry vehicles, and space vehicles, as well as in the preparation of marine deep-submergence pressure vessels and like structures. Existing technology was incapable of supplying such materials and the search to satisfy this need centered about the fabrication of composite articles.

One of the most promising materials suggested for use in composite form was high-strength, high-modulus carbon fibers, which were introduced into the market place at the very time this rapid growth in the aircraft, space and missile industries was occurring. Such fibers have long been incorporated into plastic matrices to produce composites having extraordinary high-strengthand highmodulus-to-density ratios, and more recently efforts have centered upon the preparation of composites having metal matrices.

Since nickel readily adheres to carbon and these two materials are light weight and essentially nonreactive with each other, the use of high strength, high modulus carbon fibers as a reinforcing agent for nickel in forming composites having high modulus-to-density ratios and high strength-to-density ratios has been suggested. Composites of this type, however, are generally employed in environments where they are subjected to wide ranges of temperature and, because of marked differences between the coefficients of thermal expansion of carbon and nickel, such composites have been found to possess poor thermal cycling characteristics. Under such conditions, these composites undergo extensive, irreversible dimensional distortions in a direction perpendicular to the fiber axis, which is accompanied by the formation of considerable porosity and a significant degradation of the mechanical properties of the composite, such as Youngs modulus and flexural strength. Thus, for example, when repeatedly heated from room temperature to 500 C., composites containing 46 percent by volume of carbon fibers and 54 percent by volume of nickel were found to increase in porosity by 37 percent, while the Youngs modulus and flexural strength of the composites decreased from 43x10 p.s.i. and 140x10 p.s.i. to 23x10 p.s.i. and 20x10 p.s.i., respectively.

The irreversible dimensional deformation of a material induced by thermal cycling is known as ratcheting. It is believed that this effect, and the disintegration of composite properties which accompany it in carbon fibernickel matrix composites, is a result of shear stresses which develop at the interface of the carbon fibers and the matrix of the composite as it is heated to high temperatures and subsequently cooled. Thus, as the composite temperature increases as it is heated, the interfacial bond strength between the carbon fibers and the nickel matrix also increases while, at the same time, the nickel matrix expands at a greater rate than do the fibers. Full expansion of the matrix at the fiber interface is constrained, however, by the interfacial bond strength of these materials. This causes a compressive stress to be exerted on the matrix and a tensile stress to be exerted on the fibers. Ultimately, the compressive stress on the matrix is relieved by plastic flow of the matrix in a direction perpendicular to the fiber axis, resulting in distortion of the composite dimensions.

When the heating cycle is reversed and the composite is cooled from an elevated temperature, the stresses exerted on the matrix and fibers are reversed, i.e., the fibers are subjected to a compressive stress and a tensile stress is exerted on the matrix as it attempts to contract at a faster rate than the interfacial bond strength with the fibers will allow. At the same time, however, the interfacial bond strength of the fibers and matrix is gradually reduced by the cooling and, eventually, declines to a point where it is exceeded by the tensile stress exerted on the matrix. This causes slippage of the matrix and a permanent weakening of the bonding between the fibers and the matrix, which seriously degrades the mechanical properties of the composite.

SUMMARY OF THE INVENTION In accordance with the instant invention, it has now been discovered that the interfacial bonding characteristics of carbon fiber-nickel base metal matrix composites can be substantially improved by providing a thin, intermediate layer of a metal carbide at the interface between the surface of the fibers and the nickel base matrix. As a result of this improved interfacial bonding between the fibers and matrix, the composites are characterized by improved thermal cycling characteristics and may be repeatedly cycled over a wide range of temperatures without undergoing the ratcheting effect and mechanical property degradation heretofore characteristic of carbon fiber-nickel matrix composites. Thus, even after being thermally cycled 500 times over a temperature range of from C. to 500 C., no ratcheting is observed in composites prepared in accordance with the present invention.

The improved carbon fiber-nickel base metal matrix composites of the present invention are formed by immersing a plurality of high strength, high modulus carbon fibers in a molten metal bath containing an alloy consisting essentially of at least one metal capable of reacting with carbon to form a metal carbide and at least one acidsoluble metal which does not react with carbon to form a metal carbide and which acts as a vehicle or carrier for the carbide-forming metal; soaking the fibers in said molten metal bath for a time sufficient to effect reaction between the fibers and the carbide-forming metal to produce a metal carbide coating on the surface of the fibers; re moving unreacted metal from the metal carbide coated fibers so-produced; and incorporating the fibers, preferably in a side-by-side or parallel manner, into a nickel base metal matrix.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Any metal carbide which can be formed by the reaction of carbon and a metal capable of reacting with carbon to produce a metal carbide can be employed to improve the interfacial bonding characteristics of carbon fiber-nickel base metal matrix composites according to the present invention. Coatings of this type may be provided on the surface of a carbon fiber by immersing the fiber in a metal capable of reacting with carbon to form a metal carbide for a time sufiicient to effect reaction between the fiber and said metal. However, in order to prevent excessive degradation of the fiber, reaction between the fiber and metal is effected in the presence of a diluent. In practice, this diluent takes the form of a metal which is capable of alloying with the carbide-forming metal, but which itself does not react with carbon to form a metal carbide. Formation of the alloy lowers the reaction temperature between the fiber and the carbideforming metal and allows the formation of a thin, continuous, non-granular coating.

Conveniently, an alloy consisting essentially of at least one metal capable of reacting with carbon to form a metal carbide and at least one acid-soluble metal which is capable of alloying with said carbide-forming metal and which itself does not react with carbon to form a metal carbide can be employed to provide a metal carbide coating on the surface of the fibers. The alloy is heated to a temperature which is both sufliciently elevated to render it molten and sufficiently elevated to effect reaction between the carbon fibers and the carbide-forming metal, following which the carbon fibers are immersed in the molten alloy for a time sufficient to effect reaction between the fibers and said carbide-forming metal. When reaction between the fibers and metal is complete, the fibers are removed from the molten alloy and, after cooling, immersed in an acidic solution to dissolve unreacted acid-soluble metal present on the fibers.

The alloy employed in the process of the present invention, as previously stated, consists essentially of at least one metal which is capable of reacting with carbon to form a metal carbide and at least one acid-soluble metal which is capable of alloying with such carbideforming metal and which itself does not react with carbon to form a metal carbide. Among the metals capable of reacting with carbon to form a metal carbide are titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten and iron. Among the acid-soluble metals which do not react with carbon to form a metal carbide are indium, germanium, gallium, tin, antimony, bismuth, silver, copper, and the like. Most preferably, titanium is employed as the carbide-forming metal and indium is employed as the carrier metal.

In order to ensure formation of sufficient metal carbide to provide a continuous coating on the carbon fibers, the carbide-forming metal must be present in the alloy in an amount of at least 0.1 percent by weight. Since it is undesirable to effect reaction between the carbon fibers and carbide-forming metal which is unalloyed, the carbide-forming metal is preferably not present in the bath in excess of the amount which will form an alloy with the carrier metal. In any event, any unalloyed carbideforming metal that is present should not exceed 5 percent by weight of the total weight of metals present. The amount of alloyed carbide-forming metal present should be limited to no more than 5 percent of the total weight of the alloy so as to prevent excessive reaction with the carbon fibers. Preferably, the amount of alloyed carbideforming metal present is from 0.2 percent by weight to 1 percent of the total Weight of the alloy. The carrier metal, which makes up the balance of the alloy, is thus present in an amount of from percent by weight to 99.9 percent by weight, preferably from 99 percent by weight to 99.8 percent by weight, when no unalloyed carbide-forming metal is present. When unalloyed carbide-forming metal is present, the carrier metal may be present in an amount of from at least 90.4 percent by weight up to less than 99.9 percent by weight, preferably from at least 94.3 percent by weight up to less than 99.8 percent by weight, of the total weight of metals present. When the metals employed are titanium and indium, the titanium is preferably present in an amount of from 0.2 percent by weight to 0.6 percent by weight and the indium is present in an amount of from 99.4 percent by weight to 99.8 percent by weight of the total weight of such metals.

In order to effect reaction between the carbon fibers and the carbide-forming metal, it is necessary to heat the alloy to a temperature which is both sufiiciently elevated to render it molten and sufiiciently elevated to effect the desired reaction. Such temperatures will, of course, depend upon the specific metals present in the alloy. Generally, temperatures of from about 600 C. to about 1200 C., preferably from about 800 C. to about 1100 C., are suitable. Because the carbide layer forms at relatively low temperatures and the reaction takes place in the presence of a diluent metal, the mechanical properties of the fibers are not degraded by the process.

The time required to effect reaction between the carbon fibers and carbide-forming metal depends upon the particular metal employed, the amount of such metal present in the alloy, and the temperature at which reaction is elfected. Under most circumstances, reaction can be completed within from about 10 seconds to about 300 seconds, usually within from about 60 seconds to about 180 seconds. Since prolonged reaction periods and/ or excessive heating may cause some degradation of the fiber properties, it is preferred that the reaction time not exceed 240 seconds and that the temperature employed be not greater than 1100 C.

After reaction between the carbon fibers and carbideforming metal is complete, the carbon fibers are removed from the molten alloy and, after cooling, immersed in an acidic solution to dissolve unreacted acid-soluble metal present on the fibers. Any mineral acid, i.e., hydrochloric acid, nitric acid or sulfuric acid, can be employed. The solution usually contains from 10 percent by volume to percent by volume, preferably from 20 percent by volume to 50 percent by volume, of such acid, and is employed in an amount sufficient to completely dissolve any unreacted acid-soluble metal present on the fibers, for example, from about 0.5 cc. to about 20 cc. of the solution per gram of unreacted acid-soluble metal should be employed. Heating is usually unnecessary as the metal readily dissolves in the acid without heating.

For convenience, the fibers may be wrapped around a spool or similar object before being immersed in the acid solution. The fibers should be allowed to soak until all the unreacted acid-soluble metal present has been dissolved. The duration of the soaking period depends upon the nature and concentration of the acid, the particular metal present on the surface of the fibers, the amount of such metal, and the temperature employed. In order to fully remove all the unreacted acid-soluble metal from the surface of the fibers, it is usually necessary to soak the fibers for at least minutes, most usually from minutes to 120 minutes.

After the carbon fibers have been immersed in the acid bath for a time sufficient to remove unreacted acidsoluble metal from the fibers, they are removed from the bath, washed with water to remove acid, and dried. Any excess carbide-forming metal present which failed to react with the fibers may be easily brushed from the fibers. The fibers may then be incorporated into a nickel base metal matrix, in accordance with known techniques, to produce unique composites having improved thermal cycling characteristics.

One method of incorporating the metal carbide coated fibers into a nickel matrix is by depositing nickel on the fibers, and then hot pressing the nickel coated fibers to bond them together. Deposition of nickel on the fibers can be effected by a variety of methods, including electrodeposition from an aqueous salt bath, electroless deposition, thermal decomposition of an appropriate metal carbonyl or halide, and sputtering. Electrodeposition from an aqueous salt bath provides a uniform, tenaciously bonded coating and is the preferred means of applying nickel to the fibers according to the instant invention. The coated fibers produced by these techniques are then bonded together, preferably in a side-by-side or parallel manner, by hot pressing in a non-oxidizing atmosphere, e.g., in an inert atmosphere or under vacuum. By an inert atmosphere is meant an atmosphere which does not react with nickel under the reaction conditions employed during hot pressing, such as nitrogen, argon, xenon, helium and the like.

Generally, hot pressing is efl ected by heating the nickel coated fibers to a temperature sufficiently elevated to cause sintering of the nickel coating and applying sufficient pressure to bond the sintered nickel coated fibers together into a composite. Unnecessary severe processing conditions should be avoided during hot pressing as this may cause physical and chemical damage to the fibers and weakening of the composite. For example, excessive temperatures may cause dissolution of the metal carbide coating into the nickel matrix, while excessive pressure may result in fiber rupture. For this reason, it is preferred to use the minimum processing conditions necessary to attain maximum densification, i.e., to eliminate virtually all porosity and produce a non-porous article.

Hot pressing of the nickel coated fibers can be readily effected at temperatures of from about 700 C. to about 1300 0., preferably from about 800 C. to about 1100 C. The pressure required will, of course, depend upon the temperature employed, with higher pressures being required at lower temperatures. Pressures in excess of 500 p.s.i. are usually employed, with pressures of from about 1500 p.s.i. to about 2500 p.s.i. being preferred. To avoid fiber rupture during hot pressing, it is preferred not to use pressures in excess of 4500 p.s.i.

Hot pressing should be continued for a time sutficient to achieve effective bonding of the coated fibers and to attain maximum densificaiton. The time required to accomplish this will depend, of course, upon the temperature and pressure employed. When a temperature of 1050 C. and a pressure of 3500 p.s.i. is employed, about 45 minutes is required to eliminate virtually all porosity; at 1050 C. and 3000 p.s.i. pressure, about 60 minutes is required; and at 950 C. and 3500 p.s.i. also about 60 minutes.

Where it is desired to produce composites containing carbon fibers in a matrix containing metals in addition to nickel, a second coating of another metal may be applied to the nickel coated fibers before the hot pressing step. The dual-coated fibers are then hot pressed for a time and at a temperature sufficient to bond the fibers together and diifuse the second coating into the nickel.

An alternative method of producing composites having a matrix containing metals in addition to nickel, is by hot pressing the metal carbide coated fibers between thin nickel base metal foils.

When hot pressing is undesirable, composites may be formed by electroforming techniques.

High modulus, high strength carbon fibers suitable for use in the instant invention can be prepared as described in U.S. Letters Patent 3,503,708 and 3,412,062.

The following example is set forth for purposes of illustration so that those skilled in the art may better understand this invention, and it should be understood that it is not to be construed as limiting this invention in any manner. The term carbon as used throughout this specification includes all forms of the material, both graphitic and non-graphitic. The term nickel base metal matrix includes matrices containing at least 10 percent by weight of nickel.

EXAMPLE A molten metal bath containing 99.5 percent by weight indium and 0.5 percent by weight titanium was prepared by heating indium and titanium, in the desired ratio, to a temperature of 850 C. under argon in a graphite crucible. A two ply graphite yarn having 720 filaments per ply wherein the filaments are characterized by an average Youngs modulus of 75 x 10 p.s.i. and an average tensile strength of 335 X 10 p.s.i. was then immersed into the molten alloy for 4 minutes to allow the molten metal to completely infiltrate the fiber bundle and react with the fibers to produce a thin titanium carbide coating on the surface of each of the fibers.

After being removed from the molten alloy, the fibers were cooled to room temperature in argon, and then immersed in a 50 volume percent aqueous solution of hydrochloric acid and allowed to soak for about 15 minutes in order to dissolve all the indium metal present on the fibers. At the end of this time, the fibers were removed from the acid solution, washed in water, and dried. Metallographic examination of the resultant fibers showed a thin coating of titanium carbide, about 0.5 micron thick, present on the surface of each of the fibers. The coated fibers had an average Youngs modulus of 70 l0 p.s.i. and an average tensile strength of 302x10 p.s.i.

The titanium carbide coated fibers were then electroplated with nickel using a standard Watts plating solution. Two inch lengths of the fibers were clamped to an electrical lead and immersed in a plating bath where nickel was deposited on the fibers from a nickel anode. The aqueous electroplating solution employed contained 200 grams of NiSO -6H O, 50 grams of NiCl -6H O and 30 grams of H BO per liter of water. The solution was maintained at room temperature, and a plating current of 0.75 ampere was employed. At the end of 4 minutes, the electrical lead was removed and attached to the other end of the fibers, and the procedure was re peated. The yarn was then removed from the bath, washed in hot water, and dried at 200 C. Metallographic exami nation of the resultant nickel coated fibers showed that all monofilaments had a coating of nickel thereon, and that the coating thickness ranged from 1 to 3 microns.

The nickel clad fibers were then placed in a fused quartz ampoule. Air was evacuated from the ampoule, and the ampoule was then pressurized with argon to a pressure of about one-half atmosphere and sealed. The ampoule and its contents were then thermally cycled 500 times between C. and 500 C. At the end of this time, the nickel clad fibers were examined and found to exhibit no dimensional change as a result of the thermal cycling. Metallographic examination showed the nickel was still uniformly clad around the filaments. On the other hand, nickel clad fibers produced from identical fiber, in the same manner, but without the prior application of a titanium carbide coating, had many separations and voids between the fibers and the surrounding nickel.

Composites produced in accordance with the invention are extremely useful as materials of construction for subsonic and supersonic aircraft, space system components and various propulsion devices. In addition to use in preparing such composites, metal carbide coated carbon filaments are useful as oxidation-resistant heating elements.

It will be readily apparent to those skilled in the art that the composite articles of the instant invention may be fabricated to meet various design requirements as to size, shape, stress relationships, and the like. Thus, any shape in which the metallized carbon fibers are wound or stacked in parallel relationship can be provided. For example, such fibers can be wound on a mandrel and hot pressed to produce a coiled composite, or laid out in parallel manner and compressed to produce plates of various sizes and shapes. In addition, where more isotropic physical properties are desired, laminates can be prepared in which the metallized fibers are arranged in layers wherein the fibers of each layer remain in parallel relationship but are in non-parallel relationship to the fibers of the adjacent layers.

I claim:

1. A process for producing a composite article having a nickel base metal matrix reinforced with a plurality of high strength, high modulus carbon fibers having a thin, intermediate layer of a metal carbide disposed on the fiber surfaces which comprises immersing a plurality of high strength, high modulus carbon fibers in a molten metal bath containing an alloy consisting essentially of at least one metal capable of reacting with carbon to form a metal carbide and at least one acid-soluble metal which does not react with carbon to form a metal carbide; soaking the fibers in said molten metal bath for a time sufiicient to eflfect reaction between the fibers and the carbideforming metal to produce a metal carbide coating on the surface of the fibers; immersing the metal carbide coated fibers in an acidic solution to dissolve unreacted acidsoluble metal present on the fibers; and incorporating the fibers into a nickel base metal matrix.

2. A process as in claim 1 wherein the carbide-forming metal is present in the alloy in an amount of at least 0.1 percent by weight and any unalloyed carbide-forming metal present does not exceed percent by weight of the total weight of metals present; and the molten metal bath is maintained at a temperature of from about 600 C. to about 1200 C.

3. A porcess as in claim 2 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

4. A process as in claim 3 wherein the fibers are soaked in the molten metal bath for no more than 240 seconds.

5. A process as in claim 4 wherein the nickel base metal matrix is nickel.

6. A process as in claim 2 wherein the carbide-forming metal is present in the alloy in an amount of from 0.1 percent by weight to 5 percent by weight.

7. A process as in claim 6 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

8. A process as in claim 7 wherein the fibers are soaked in the molten metal bath for no more than 240 seconds.

9. A process as in claim 8 wherein the nickel base metal matrix is nickel.

10. A process as in claim 6 wherein the carbide-forming metal is present in the alloy in an amount of from 0.2 percent by weight to 1 percent by weight.

11. A process as in claim 10 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

12. A process as in claim 11 wherein the fibers are soaked in the molten metal bath for no more than 240 seconds.

13. A process as in claim 12 wherein the nickel base metal matrix is nickel.

14. A process as in claim 1 wherein the carbide-forming metal is present in the alloy in an amount of from 0.1 percent by weight to 5 percent by weight; and the molten metal bath is maintained at a temperature of from about 600 C. to about 1200 C.

15. A process as in claim 14 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

16. A process as in claim 15 wherein the fibers are soaked in the molten metal bath for no more than 240 seconds.

17. A process as in claim 16 wherein the nickel base metal matrix is nickel.

18. A process as in claim 14 wherein the carbide-forming metal is present in the alloy in an amount of from 0.2 percent by weight to 1 percent by weight.

19. A process as in claim 18 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

20. A process as in claim 19 wherein the fibers are soaked in the molten bath for more than 240 seconds.

21. A process as in claim 20 wherein the nickel base metal matrix is nickel.

22. A process as in claim 1 wherein the carbide-forming metal is titanium and the acid-soluble metal is indium, and the titanium is present in an amount of from 0.2 percent by weight to 0.6 percent by weight of the total weight of such metals.

23. A process as in claim 22 wherein the molten metal bath is maintained at a temperature of from about 600 C. to about 1200 C.

24. A process as in claim 23 wherein the fibers are soaked in the molten metal bath for from about 10 seconds to about 300 seconds.

25. A process as in claim 24 wherein the fibers are soaked in the molten metal bath for from about 60 seconds to about 240 seconds.

26. A process as in claim 22 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

27. A process as in claim 26 wherein the acidic solution is a solution of hydrochloric acid.

28. A process as in claim 26 wherein the fibers are soaked in the molten metal bath for from about 10 seconds to about 300 seconds.

29. A process as in claim 28 wherein the nickel base metal matrix is nickel.

30. A process as in claim 28 wherein the acidic solution is a solution of hydrochloric acid.

31. A process as in claim 29 wherein the acidic solution is a solution of hydrochloric acid.

32. A process as in claim 26 wherein the fibers are soaked in the molten metal bath for from about 60 seconds to about 240 seconds.

33. A process as in claim 32 wherein the nickel base metal matrix is nickel.

34. A process as in claim 32 wherein the acidic solution is a solution of hydrochloric acid.

35. A process as in claim 33 wherein the acidic solution is a solution of hydrochloric acid.

36. A process for producing a metal carbide coated carbon fiber which comprises immersing a high strength, high modulus carbon fiber in a molten metal bath containing an alloy consisting essentially of at least one metal capable of reacting with carbon to form a metal carbide and at least one acid-soluble metal which does not react with carbon to form a metal carbide; soaking the fiber in said molten metal bath for a time sufiicient to effect reaction between the fiber and the carbide-forming metal to produce a metal carbide coating on the surface of the fiber; and immersing the metal carbide coated fiber in an acidic solution to dissolve unreacted acid-soluble metal present on the fiber.

37. A process as in claim 36 wherein the carbide-forming metal is titanium and the acid-soluble metal is indium, and the titanium is present in an amount of from 0.2 percent by weight to 0.6 percent by weight of the total weight of such metals.

38. A process as in claim 37 wherein the molten metal bath is maintained at a temperature of from about 800 C. to about 1100 C.

39. A process as in claim 3-8 wherein the fiber is soaked in the molten metal bath for from about 60 seconds to about 240 seconds.

40. A process as in claim 39 wherein the acidic solution is a solution of hydrochloric acid.

References Cited UNITED STATES PATENTS ALFRED L. LEAVI'IT, Primary Examiner M. 'W. BALL, Assistant Examiner US. Cl. X.R.

15 75-DIG1; 117114R, 11s 

